The lithium ion is widely used in the treatment of affective disorders. Since it appears to have prophylactic value in manic-depressive (bipolar) illness and, perhaps, other types of affective disorders, substantial numbers of patients are being chronically treated with this agent. Lithium has a variety of effects on renal function in humans, but until recently these were thought to be reversible following the cessation of lithium treatment. However, recently there have been reports that the renal concentrating defect produced by lithium persists in some patients who have discontinued the drug. Of even greater concern are several case reports of patients on lithium maintenance therapy who have been shown to have an interstitial nephritis and impaired renal function. Since none of the case reports included renal function studies prior to the initiation of lithium therapy it is not possible to definitely ascribe a causal role to lithium. We propose to study renal function prospectively in 180 patients maintained on lithium carbonate. An extensive battery of pre-lithium baseline renal function studies will be done and repeated at monthly, six-month and yearly intervals. Patients will be followed for 2-5 years. These assessments are designed to monitor the presence or absence of all other known causes of interstitial nephritis. The patients will be used as their own controls and pre and post-lithium values compared. In this way we should be able to document the development of renal impairment in patients on lithium and whether or not other known causes of chronic interstitial nephritis were present. We should also be able to provide data on the incidence of significant renal impairment in patients maintained on lithium therapy. Other factors such as length of time on lithium, degree of concentrating defect, presence or absence of previous episodes of lithium toxicity, etc. can be examined to see if they correlate with the development of renal impairment.